The present invention relates to a method and device for converting a screen aspect ratio of a television, and more particularly to a method and device for converting a screen aspect ratio of a television which is suitable for displaying a video signal having a screen aspect ratio of 4:3 through an image device having a screen aspect ratio of 16:9.
Conventionally, when a video signal having a screen aspect ratio of 4:3 is displayed in an image device having a screen aspect ratio of 16:9, a quarter of the scanning lines are removed in the vertical direction, or, the video signal is displayed without any processing, as is. However, in the former, much picture information is lost, and in the latter, the image is distorted by expansion in the horizontal direction.
FIGS. 1A to 1C show that pictures are displayed by the conventional screen aspect ratio converting modes.
That is, one of the conventional modes (i.e., a normal mode) uses only the central three-quarters portion (or four-fifths) of a screen having an aspect ratio of 16:9 (or 5:3) and leaves right and left side panels (shaded portion) as blank space, as shown in FIG. 1A.
In this mode, as shown in FIG. 2, time-compression is performed in a horizontal direction of screen (time-compression of 3/4 is performed in conversion into a 16:9 screen aspect ratio and time-compression of 4/5 is performed in conversion into a 5:3 screen aspect ratio), and the remaining 1/4 (or 1/5) horizontal interval corresponds to the blank side panels 21 and 22. At this time, the horizontal scanning interval of the video signal having a 4:3 screen aspect ratio is the same as that of the video signal having a 16:9 screen aspect ratio (in case of NTSC type, about 63.5 .mu.sec).
However, in this case, since a quarter (or a fifth) of the screen having an aspect ratio of 16:9 (or 15:9) is used as panels, presence of image is reduced.
FIG. 1B shows a picture displayed by another one of the conventional modes (i.e., zoom mode), which uses only 3/4 (or 4/5) of the overall scanning lines of an image having a 4:3 screen aspect ratio, and performs interpolation in a vertical direction, and then displays a image on a screen having a 16:9 (or 5:3) aspect ratio.
In this mode, as shown in FIG. 3, when the video signal having a screen aspect ratio of 4:3 is converted into that of 16:9, only (3/4)N of all the scanning lines N are taken, and then are interpolated by 4:3 in a vertical direction, thereby scanning the image on the screen having an aspect ratio of 16:9.
Accordingly, in this mode, during conversion into the screen aspect ratio of 16:9, picture information corresponding to a quarter of all the picture information is lost (for instance, in case of NTSC type, 120 lines are removed from the active scanning lines, i.e., about 480 lines). And, during conversion into the screen aspect ratio of 5:3, a fifth of the picture information is lost (for instance, in case of NTSC type, 96 lines are removed from the active scanning lines, i.e., about 480 lines).
Meanwhile, another conventional mode (default mode) displays an image having a screen aspect ratio of 4:3 on a screen of 16:9 (or 15:9) without any extra processing, as shown in FIG. 1C. In this mode, since the picture is expanded by 4/3 times (or 5/4 times) in a horizontal direction, the converted image is widened by 33.3% (or 25%) in a horizontal direction with respect to the original form.